The near Earth space environment, also known as a geospace environment, or “geospace regions” and/or simply “geospace” is characterized by several regions according to their plasma properties. The geospace and/or near Earth space environment includes particles of the solar wind, the outer and inner magnetosphere, the plasmasphere, and the thermosphere, including the ionosphere.
“Not only does the Sun radiate the light we see . . . [it] blows a huge bubble of supersonic plasma . . . which engulfs the planets and a host of smaller bodies, shaping their environments. It also conveys perturbations that can be seen in our daily life”, according to N. Meyer-Vernet, Basics of the Solar Wind: “The Wind from the Sun”, Cambridge University Press 2007, p. 1 (http://www.lesia.obspm.fr/perso/nicole-meyer/BSW/BSWchap1.pdf) [Internet, accessed on May 5, 2009].
The solar wind consists of “ionized plasma, mainly protons and electrons . . . . There are strong magnetic fields on the Sun and as the solar wind moves through them, currents are induced and . . . [the] particles carry the field . . . [in the plasma]”, according to Ratcliffe, An Introduction to the Ionosphere and Magnetosphere, (Cambridge University Press, 1972), p. 13.
The primary driver of geo effective events that can disrupt space based systems (e.g., communication and navigation) are solar eruptions such as coronal mass ejections and solar flares. These disturbances propagate through interplanetary space in the solar wind and impinge upon the Earth's magnetic field thereby affecting the entire near Earth space environment. A major thrust today in space physics research is to consistently model this environment, based on physics. The goal has been to acquire the capability to predict, by direct observation and measurement, and to acquire the capability to assess in real time the impact of major solar events on military and civilian space based systems and personnel.
The taxonomy of geospace regions can be seen in FIG. 1, along with some parameters of the physical state of geospace regions. The lowest altitude region of the geospace regions, called the ionosphere, is embedded in a complex neutral gas (the thermosphere) of highly varying temperature, density and composition. The ionosphere ranges from about 1.015 Earth radii to about 1.157 Earth radii (Re), where 1 Re=6371 km approximately (thus, the ionosphere can range from about 90 km to approximately 1000 km). Above the ionosphere resides the plasmasphere, which is the interface region between the ionosphere and the magnetosphere. The plasmasphere, approximately the region of closed magnetic field lines, extends to a range of from about 2 Re to about 6 Re, depending on the level of geomagnetic activity (the higher the activity, the smaller the plasmasphere). The magnetosphere is an elongated region having an interface with the solar wind; this interface is known as the bow shock and is about 13 Re on the sunward side of the Earth, with a “tail” that extends beyond the orbit of the Moon, at 60 Re. The magnetosphere ranges from about 5 Re to about 13 Re on the Sun side of the Sun-Earth line and the magnetosphere ranges to about 5 Re to about 60 Re on the anti-Sun side, where this extended range on the anti-Sun side contributes to the above mentioned tail formation.
The boundaries of the ionosphere, plasmasphere and magnetosphere are highly variable depending on the solar wind strength and direction. The size and shape of the magnetosphere are determined by pressure balance between the solar wind plasma and the geomagnetic field. The magnetosphere is populated with charged particles that originate in both the ionosphere and the solar wind. Geospace regions are coupled by the geomagnetic field and various electric fields that are generated by the passing solar wind as well as the rotating ionosphere and other processes in the thermosphere. Large scale currents flow among the geospace regions. All geospace regions are highly variable, responding rapidly to changes in solar wind dynamics, the solar extreme ultraviolet (EUV), and X-ray radiative output. As a result, “space weather” assessment and forecasting, unlike that at ground level, depends directly on knowing and forecasting conditions on the Sun.
Space weather affects any operational system that utilizes propagation of electromagnetic waves. Electromagnetic systems include communication, navigation, position location, satellite operations, and radar. Other areas strongly impacted by space weather are astronaut safety, spacecraft charging (i.e., spacecraft receiving electric charges), operations and radiation damage, infrastructure effects, including ground level power transmission, pipeline currents, telephone and aviation communications, Global Positioning System (GPS), and tracking of space objects including debris.
The ability to assess and forecast tropospheric weather improved dramatically, when new imaging devices were flown onboard satellites at the beginning of the space age. Geospace data, on the other hand, mostly come from ground and space based in situ or transmission path sensor systems. Also, geospace data can be derived from geospace imaging that measures electron density indirectly using helium ion emissions in the plasmasphere or energetic neutral atoms (ENAs) from the inner magnetosphere ring current. Because the volume of geospace ranges from about 5 to 6 orders of magnitude larger than the volume of the troposphere, the need for a global imaging capability is even greater for the understanding of space weather and to predict its impact on various operational systems and assets, both military and civilian.
“Even though the idea . . . [of a solar wind] is an ancient one, most of the solar wind story took place over little more than a century. At the end of the nineteenth century, only a couple of far-seeing scientists had imagined that a solar wind might exist. At the beginning of the twenty-first century, hordes of space probes have explored the solar wind . . . ”, according to N. Meyer-Vernet, Basics of the Solar Wind: “The Wind from the Sun”, Cambridge University Press 2007, p. 2 (http://www.lesia.obspm.fr/perso/nicole-meyer/BSW/BSWchap1.pdf) [Internet, accessed on May 5, 2009].
In 1892, “George Fitzgerald suggested that . . . ‘matter starting from the Sun with the explosive velocities we know possible there, and subjected to an acceleration of several times solar gravitation, could reach the Earth in a couple of days’”, according to N. Meyer-Vernet, Basics of the Solar Wind: “The Wind from the Sun”, Cambridge University Press 2007, p. 3 (http://www.lesia.obspm.fr/perso/nicole-meyer/BSW/BSWchap1.pdf) [Internet, accessed on May 5, 2009].
In the early 1900's a Norwegian physicist, Kristian Birkeland “worked on three fronts: theory, laboratory experiments with a model Earth, and observation . . . . He submitted . . . that since auroral and geomagnetic activity was[sic] produced by solar particles and was[sic] virtually permanent, the inescapable conclusion was that the Earth environment was bombarded in permanence by ‘rays of electric corpuscles emitted by the Sun’”, according to according to N. Meyer-Vernet, Basics of the Solar Wind: “The Wind from the Sun”, Cambridge University Press 2007, p. 4 (http://www.lesia.obspm.fr/perso/nicole-meyer/BSW/BSWchap1.pdf) [Internet, accessed on May 5, 2009].
“Put in modern terms, Birkeland was suggesting that the Sun emits a continuous flux of charged particles filling up interplanetary space: nearly our modern solar wind” (see, N. Meyer-Vernet, Basics of the Solar Wind: “The Wind from the Sun”, Cambridge University Press 2007, p. 5 (http://www.lesia.obspm.fr/perso/nicole-meyer/BSW/BSWchap1.pdf) [Internet, accessed on May 5, 2009]).
The first “most successful of the Russian spacecraft, Lunik II launched in 1959 . . . detected a flux of positive ions . . . . The ultimate proof came in 1962 from the American spacecraft Mariner 2 . . . which was en route for Venus after having . . . survived . . . failures . . . Mariner 2 . . . [identified] general properties of the solar wind” (see, N. Meyer-Vernet, Basics of the Solar Wind: “The Wind from the Sun”, Cambridge University Press 2007, p. 7 (http://www.lesia.obspm.fr/perso/nicole-meyer/BSW/BSWchap1.pdf) [Internet, accessed on May 5, 2009]).
In 1972, the “first optical-astronomy observations from the lunar surface were made by the Apollo 16 astronauts . . . . These observations were made with the Naval Research Laboratory's far-ultraviolet camera/spectrograph (Experiment S-201) . . . . Some of the specific objectives of this experiment were . . . studies of the [E]arth's upper atmosphere, geocorona, and magnetosphere, and their interaction with the solar wind . . . . The instrument was an . . . electronographic Schmidt camera . . . [the experiment and instruments were designed by Dr. George R. Carruthers of the Naval Research Laboratory (NRL)] the camera effectively viewed a rectangular area of the sky having dimensions ½ degree by 20 degrees . . . the [E]arth was pointed at directly . . . “[by Apollo 16 mission commander John W. Young, using the electronographic Schmidt camera]” (see, Carruthers, “Apollo 16 Far-Ultraviolet Camera/Spectrograph: Instrument and Operations”, APPLIED OPTICS, (October 1973), Vol. 12, No. 10, pp. 2501-2508.
It can be seen that techniques developed so far for geospace imaging have only focused on observing proxies. Thus, the electronographic Schmidt camera measured far ultraviolet spectral emissions from various atoms and molecules in the earth's atmosphere. It also measured star fields and nebulae, but did not include an occulting disk; it did not observe in the visible spectral band; it did not measure Thomson scattering of sunlight by electrons; it cannot and did not observe the magnetosphere, plasmasphere, and/or much of the ionosphere.
Additional techniques developed for geospace imaging also only focused on observing proxies, such as proxies of the electron density in the geospace, including the helium ion emission from the plasmasphere at 30.4 nm, energetic neutral atoms (ENAs) from the ring current region of the inner magnetosphere, or far ultraviolet imaging of the ionosphere and thermosphere.
Remote sensing of the plasma regions has been accomplished by actively using Extreme Ultraviolet (EUV) radiation and passively by observing the helium (He) ion distribution in the plasma regions by detecting resonantly scattered solar 30.4 nanometer (nm) ultraviolet (UV) radiation. This remote sensing technique is only a proxy for the electron density in the plasmasphere; the relationship between the electron density and the He ion density is highly variable.
In 1995, several approaches of imaging the Earth's plasma environment were evaluated. The main plasma regions are shown schematically in FIG. 1. At least two (of several) physical mechanisms were considered:
(1) A first physical mechanism considered was the outermost edge of the plasma regions where the brightness of 30.4 nm helium emissions drops off (i.e., where He+ represents the outermost edge). This edge region is called the plasmapause. The quantities imaged were not electrons, but rather helium ions in the plasmasphere. Models tacitly assumed a fixed relationship to the electron distributions. Thus, what was considered was a poor proxy for determining the electron density. This emission was discovered by a U.S. Naval Research Laboratory (NRL) rocket experiment in 1969, and had been routinely imaged extensively by the EUV experiment conducted on the National Aeronautics and Space Administration (NASA) Imager for Magnetopause to Aurora Global Exploration (IMAGE) satellite. FIG. 1 shows a typical image from the IMAGE satellite. The principal drawback of this technique is that the electron density is not measured directly; and He+, which is the proxy for electron density, is typically of the order of 10 percent of the electron density in the plasmasphere, but can be as much as fifty percent or as low as one percent, approximately.
(2) A second physical mechanism considered was radiative recombination of the H+ plus an electron (i.e., H++e) into the ground state of Hydrogen (H), resulting in a narrow continuum emission at 91.1 nm. The intensity is the integral along the line of sight of the product of the recombination rate times the electron density and the proton density; the integrand is essentially the square of the electron density. In the ionosphere, the intensity level when viewing in the vertical is of the order of a few Rayleighs, but the recombination there is with oxygen (O) ions (the continua are close in wavelength due to the nearly identical ionization potentials of H and O). The Rayleigh is a unit of measure of the perceived power of light (i.e., luminous flux) used to measure the weak emission of light (i.e., air glow), such as of an aurora). The drawback with this approach is that in the plasmasphere, estimated radiative light emission levels are of the order of millirayleighs, which would challenge detection by current EUV technology. In addition, in the magnetosphere, the signal is much lower.
Thus, the NASA IMAGE satellite, which was operational from 2000 to 2005 provided some limited imaging capability, measuring helium ions in the plasmasphere and energetic neutral atoms from the inner magnetosphere, instead of measuring electrons directly. In addition, models or tacitly assumed distributions were required to relate these measured quantities of proxies to the electron distributions. Other ground based and satellite detectors and/or sensors have imaged selected regions of the nighttime ionosphere by observing radiation produced by radiative recombination of atomic oxygen ions and electrons. The recombination intensity is proportional to the product of the electron density and the oxygen ion density.
General instrument designs for the externally-occulted coronagraphs and the heliospheric imagers have been used on previous NASA science missions to capture images of coronal mass ejections and the solar wind that propagates from the Sun. For example, the COR2 instrument on the STEREO mission (launch date: October 2006) and the C2 and C3 instruments on the SOHO mission (launch date: December 1995) are predecessor geo plasma telescopes. The Heliospheric Imager on the STEREO mission (launched by NASA in October 2006) is an earlier version of the magnetopause imager telescopes discussed herein. However, the differences presented herein involve Geocorona instruments specifically designed and tailored for the application of geospace imaging.
Thus, there are no known methods and/or systems for directly imaging electrons in the near Earth environment and to study the global shape of the magnetosphere profile.
Therefore, the need exists for a method and system of globally monitoring space weather conditions, by providing the ability for observers to view the near Earth atmosphere, where the illuminating radiation of the Earth is at a minimum, based on minimal electron densities, to determine how electrons in the magnetosphere, plasmasphere, and ionosphere are redistributed in response to solar wind, geomagnetic forcing.
Furthermore, the need exists for a method and system of globally monitoring space weather conditions, by imaging electrons directly on a global scale to understand mechanisms of solar wind plasma entry into the magnetosphere by globally imaging structures along the magnetopause and magnetospheric boundary layers and to further determine how variations of the duskside plasmasphere and plasmapause are coupled to the global dynamics of the magnetosphere.
Furthermore, the need exists for a method and system of simultaneously measuring, in the near Earth plasma environment, emission levels in the order of millirayleighs and lower.
Further, the need exists for a method and system that will provide the ability for observers to view simultaneously the electron distribution in the various plasma regions around the Earth and to view how the electron distribution responds to changes in solar conditions. Much of the physics of interest resides at the boundaries of regions, where the electron densities are expected to be lower.
Further, the need exists for a method and system that will provide images of the full coupling of all geospace regions simultaneously during periods of strongly varying solar output; thus, observing the interaction with the solar wind and the propagation of plasma along with establishing cause and effect relationships.
Further, the need exists for a method and system that will provide the ability to globally assess and forecast space weather effects (particularly at the North and South poles of the Earth), as well as assess and forecast weather and radiation effects on space operational systems and assets.
Applications and operational fields that are likely impacted by the ability to directly and globally view the variations of the geospace regions due to solar wind forcing will lead to an improvement in geospace awareness, understanding and anomaly resolution, in regard to the forecasting of satellite environments, forecasting of Global Positioning System (GPS) accuracies and outages, as well as assessment of communications capabilities.